The invention relates to a process for manufacturing a lid which is coated at least on its outer surface, in particular a painted metal lid for beverage cans, having at least one pouring outlet and such that a closure strip with sealable coating is sealed onto it at least closing off the pouring outlet, said strip being, if desired, shaped from a strip or the like coated with a sealable material.
Today, beverage cans are generally made as one piece cans from tin plate, tin-free steel or aluminum and, after filling with the desired contents, closed off with a lid likewise made from the above mentioned materials. In an earlier stage this lid is provided with an outlet opening, for example by stamping, which is then closed off again by a closure strip. The lid features a coating of paint or lacquer, for example of phenolic-epoxy resin, the closure strip e.g. of thin aluminum strip a coating of a thermoplastic plastic, for example a polyamide. The lid is placed on a pre-heated lower sealing tool and the closure strip sealed around the outlet opening by melting the polyamide sealing layer by means of an upper sealing tool.
As the strength of bonding of the sealing layer to the painted surface of the lid is of great importance for the use of the can, considerable attention must be given to this feature. The strength or resistance of this bond is influenced in particular by:
the positioning of the lid at the sealing station PA1 the flatness in the region of the opening PA1 the distribution of the compressive load on sealing PA1 the thickness and breadth of the seal PA1 the temperatures of the upper and lower sealing tools PA1 the flanging conditions PA1 the possible level of pressure inside the closed can PA1 the conditions of storing the filled can PA1 the head space on the filled can PA1 the storing conditions for the lacquer on the outside of the can lid or the like.
In view of the above factors which affect seal strength, simple sealing-on of the closure strip is not sufficient, in particular for beverage cans holding drinks containing CO.sub.2. The seal on such cans must withstand an internal pressure of more than 4 bar and temperatures in excess of 30.degree. C. over extended periods of time.
To improve the bond strength of the seal, it was suggested therefore that, after sealing the closure strip onto the lid, this should be heated to melt the polyamide and then cooled again. The heating was carried out in a temperature range of 175.degree. C. to 300.degree. C. for a length of time which depended on the temperature employed. It turned out, however, that blisters formed on re-melting the seal; these blisters seriously impaired the effectiveness of the seal and, depending on the number of blisters present even destroyed the sealing effect.
The object of the present invention is, therefore, to develop a process of the kind described above by means of which blister formation is avoided, better storage properties are obtained and the bond strength between the closure strip and the lid is improved.